Direct band-gap semiconductors, such as gallium aresenide (GaAs), are useful, in a variety of devices. Two such devices, solar cells and fast computer switches, call for material of vastly different electrical properties, including the Shockley-Read-Hall lifetime (.tau..sub.SRH) of minority carriers. In solar cells, series resistance must be minimized to maximize output current. A longer lifetime permits charge carriers to move greater distances before recombination, reducing series resistance. However, rapid current decay is important in high speed switching, and short carrier lifetimes enhance current decay.
Prior attempts to measure minority carrier lifetimes in semiconductors are described in: O. von Roos, J. Appl. Phys. 50, 3738 (1979); U.S. Pat. No. 4,122,383, O. von Roos; and Weiner, et al., J. Appl. Phys. 55, 3889 (1984).
In the methods of the von Roos publications, devices containing p-n junctions are irradiated by electron beams or monochromatic light to measure minority carrier lifetime. These methods are "destructive" of the sample in the sense that a p-n junction must be formed and ohmic contacts must be applied to measure an electrical signal. In the Weiner, et al. article, a short pulse of incoming laser light is applied to GaAs to trigger a luminescence pulse which decays in a characteristic time. Carrier lifetime is calculated from the decay time. The Weiner et al. method is nondestructive of the material, but is aperiodic and does not provide the accuracy desired in carrier lifetime measurements.
The impact of radiative recombination and the reabsorption of recombination radiation on minority-carrier transport in direct band-gap semiconductors has been explored in the following publications: O. von Roos, J. Appl. Phys. 54, 1390 (1983); and O. von Roos, J. Appl. Phys. 54, 2495 (1983). However, they do not treat the time dependent case and do not propose a means for measuring lifetime.
Therefore, it is desirable to provide a method for accurately measuring minority carrier lifetime in a homogeneous sample of a direct band-gap semiconductor, without altering the sample material by introducing a p-n junction or applying ohmic contacts to it.